1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus for controlling a developer to be replenished in accordance with the density of the developer especially in a laser printer and a copying machine of an electrophotographic type, and the like.
2. Description of the Related Art
A conventional image forming apparatus, e.g., an image forming apparatus in a laser printer or a copying machine of an electrophotographic type often uses a developing unit for developing an electrostatic latent image on a photosensitive body by a magnetic brush method using a two-component developer consisting of a toner and a carrier. In the image forming apparatus using such a developing unit, toner density control is performed in the following manner. The toner density of a developer stored in the developing unit is detected by toner density detecting means, and toner replenishing means of the developing unit is operated in accordance with the detection result. The toner is replenished upon operation of this toner replenishing means so that the toner density of the developer is always automatically controlled to be a constant value. When the replenishment toner amount of the toner replenishing means becomes smaller than a predetermined amount (toner empty), this state is detected and alarmed. More specifically, an actuator arranged in a toner hopper for storing the replenishment toner approaches a lead switch attached to a side surface of the toner hopper as the toner amount is decreased. As a result, the lead switch is turned on by a magnet attached to the distal end of the actuator. Then, a resultant ON signal turns on a toner empty display unit (TOSHIBA LASER BEAM PRINTER TN-7700, for example, U.S. Pat. No. 4,647,950).
In the above-described conventional apparatus utilizing toner empty detection, however, the lead switch is turned on before the toner in the toner hopper is completely emptied. For this reason, toner empty in the toner hopper cannot be accurately detected. In addition, it is difficult to simplify the structure of the toner hopper by forming the toner hopper into a cartridge. This causes an increase in cost. Furthermore, since detection is performed by a mechanical means, the number of required parts is inevitably increased.